The present invention relates to a method for operating an asphalt plant. More specifically, it relates to an integrated control system that permits the operator to make consistently high quality asphalt with the minimum amount of wasted material and plant downtime.
There is an increasing demand for higher quality asphalt that meets more stringent performance and composition standards and that is produced in a more eco-friendly manner. It has been reported that some asphalt producers may waste up to 20 tons of product every time they start up before they reach a steady state production of consistent quality product.
The problem can be more serious if the manufacturing process goes off-spec during a production run, resulting in the production of a large amount of asphalt which does not meet the required specification. This product may be rejected by the end user or, if accepted despite not meeting the standard, it may incur a price penalty for having fallen short of the specification. If the product is rejected by the intended user, it may have to be sold at a steeply discounted price to a third party user or totally discarded as waste.
Another problem is incurred if the plant must be shut down due to a component failure or process failure. This is typically referred to as a hot stop. If the issue can be resolved quickly, the plant may attempt a hot start in which all motors and systems are started in unison so as to maintain the proper, pre-stop setting of raw material ratios in an effort to minimize the amount of off-spec asphalt produced. On some asphalt plants, this simultaneous startup of all motors, drives, and systems can put an unbearable starting load on the plant's power system, potentially causing a new shutdown even before the plant is back up on line. If it takes too much time to resolve any issue, the plant will have to proceed with a cold start-up with all the inherent time and material waste of such a start-up.
Naturally, it is desirable to minimize the amount and duration of down time to correct any issues. Preferably the issue can be identified immediately, with the decision to shut down the facility being taken only when it has been established that the issue is real (that is, it is not a false alarm).
An asphalt plant may produce several different grades of asphalt depending on the customer or the application. Change-over to a different grade or formulation has also been a source of much wasted product. Typically, once the run for one formulation has been completed, the flow to the finished product silo is shut off, the ratios of raw material inputs are changed one-at-a-time by the operator according to the new formulation, and the plant is run until steady state is achieved and the plant is once again producing on-spec asphalt. Samples of the asphalt exiting the plant are taken to a laboratory to test its characteristics and composition. If the tests come back showing that the asphalt being produced is not within the specifications, then all that asphalt is out of spec and cannot be sold to the customer within the terms of the contract. This may involve tons of wasted product.
Finally, many plants have cut back on personnel, having only two people on site to run the plant—one person operating the front-end loader and a second person operating the plant. It is very difficult for the people running the plant to do their assigned tasks while at the same time maintaining the plant and ensuring that everything is adjusted correctly and that they are making a quality asphalt mix.